1. Technical Field
The present invention relates to an instrument for measuring the mass flow rate of powder conveyed together with a gas, a constant powder feeder, which is easily manufactured, operated, maintained, and its operational efficiency is highly enhanced by applying the said instrument, and a high-performance electrostatic powder coating apparatus in which the constant powder feeder is utilized.
2. Background Art
Conventionally, constant powder feed systems for pneumatically conveyed powder as shown in FIG. 4 and FIG. 7 of the accompanying drawings are known as being used for accurately feeding an expensive powder to each of several to several tens of apparatuses, with a relatively small amount of constant flow rate, e.g., several tens to several hundreds of grams per minute, as in the case of powder feed to powder coating apparatuses, thermal spraying apparatuses and the like.
FIG. 4 shows one example of a so-called volumetric type constant powder feeder, wherein powder 102 in a hopper 101 is fluidized by means of compressed air 104 through a porous plate 103 and fed by a screw feeder 106 provided at the bottom of the hopper 101 to an injector 113 where the powder material is fed to a powder gun by means of the injector 113.
At this time, the feeder is able to feed powder with a volumetric flow rate (Fv) represented by the following equation: EQU Volumetric flow rate Fv=AR (cc/min.)
The equation is based on a constant which is determined by the volume of a screw, that is, [an effective volume per one pitch of the screw]=A (cc), and [the motor 108 rotation speed]=R/min. However, the feeding rate essentially required in the powder coating process is not a volumetric flow rate (cc/min.) but a mass flow rate (g/min.). Therefore, it is common to presume the mass flow rate by reflecting the actual bulk density S of the powder in the inside 107 of the screw feeder 106 (hereinafter referred to as feeding bulk density) using the following equation: EQU Mass flow rate FM=SAR (g/min.)
Then, the mass flow rate is presumed by graduating the feeding rate indicator 110 on the motor rotation speed.
For the volumetric type constant powder feeder, it is an important point to avoid the influence of the powder level in the powder hopper upon the feeding bulk density S. In the embodiment shown in FIG. 4, a powder level controlling device 105 is provided for this purpose. Besides the above-described one, there are some other methods used for the purpose of insulating the influence of the powder level upon the feeding bulk density, but most of them adopt a method for fluidizing powder in a hopper. This situation is the same with respect to volumetric type constant powder feeders which utilize a table feeder, an eccentric pump, a roll with grooves and the like as a feeding means.
The loosened bulk density of a fresh powder that largely affect on the feeding bulk density S varies in the wide range of 0.4 to 0.75 g/cc depending on the kind of the powder. It sometimes varies by 3 to 5% by the production lot even in the same kind of powders. The feeding bulk density S is also affected by 5 to 15% depending on the fluidizing air flow rate. The reason for this is, besides the overall problem of the powder hopper, an air flow resistance that causes fluctuation by several percentage depending on the location of the porous plate 103 in case a plurality of screws are provided to one powder hopper. Since this situation directly affects the feeding bulk density, it necessiates checking the feeding bulk density for each screw, or in other words, an adjustment of the rotation speed for each screw in relation to the rotation speed and the on-site measuring of the feeding rate, as a constant feeding apparatus which is required to have the flow rate accuracy of .+-.2.5%.
In addition, the bulk densities of the fresh powder 111 and recovered powder 112 sometimes varies in the range of 0 to 25%. This situation directly affects on the feeding bulk density when the powder is recycled as common in many powder coating lines. Moreover, since the effect of the mixing ratio of the fresh powder and recovered powder is revealed through the transfer efficiency of the coating line, it is very difficult to maintain the accuracy of the feeding bulk density, or the accuracy of the feeder within .+-.2.5% especially in a multi-purpose coating line in which a variety of objects to be coated are handled, wherein the transfer efficiency thereof tends to vary.
The mechanism and problems of the volumetric type constant powder feeder are summarized in FIG. 5. FIG. 6 shows the relation numerically. When the screw rotation speed is set to be constant, the relation between the volumetric flow rate and mass flow rate varies in a wide range of the area indicated between the lines 115 and 116. When taking into consideration of the influence of the state of fluidization and transfer efficiency of the coating line, probably the values vary in a wider range of the area indicated between the lines 118 and 117. For the purpose of determining the feeding bulk density that prescribes the volumetric flow rate, i.e., the relation between the rotation speed of the screw and the actually required mass flow rate, there is nothing for it but to regulate the actual flow rate of the practically used powder for each feeder, or for each screw in case of a multiple feeder, at a working site.
While a long period of working time and proficiency are required for this regulating work, it is not easy to always maintain a feeding accuracy of about .+-.2.5%. Thus, it is the first crucial problem common in volumetric type constant powder feeders. The volumetric type constant powder feeder, which can not be operated without regulating each apparatus individually depending on the practical working conditions on-site, can be hardly recognized as an industrial measuring and controlling equipment, therefore the creation of a new system that enables a direct detection and control of the mass flow rate of powder has been waited.
The screw feeder 106 as an apparatus having a volumetric type constant powder feeding function (hereinafter referred to as a volumetric type constant powder feeder) shown in FIG. 4 has such as complicated structure that the screw feeder requires dismounting and disassembling for having a cleaning at the time of color change of the powder together with the hopper. This requires a long period of working time and many hands. When a prompt color change is required, spare machines have to be prepared, which calls for a large amount of investment of equipment. The said difficulty in dealing with color change is the second problem of a volumetric type constant powder feeder. The same is true with respect to other systems which utilize a volumetric type constant powder feeder other than a screw feeder.
The third problem of the volumetric type constant powder feeder is that the screw feeder shown in FIG. 4 and other volumetric type constant powder feeding apparatuses are always large in size, heavy in weight, and expensive without exception. Furthermore, the space between neighboring volumetric type constant powder feeding apparatuses becomes so large that hoppers are required to be strongly constructed in a square shape with a high accuracy. As hoppers tend to be expensive, heavy and large-sized, they are inconvenient to carry, thereby hinder the operations such as color change at working sites. It is also practically impossible to streamline the coating process inexpensively by adding the above-mentioned volumetric type constant powder feeder to the hoppers of already installed powder coating facilities.
FIG. 7 shows another example of the conventional art other than the above-mentioned volumetric type constant powder feeder. A desired powder feeding rate is obtained by the steps comprising: flowing a measuring gas 114 at a constant velocity into a measuring tube 120 by means of a nozzle 121; introducing a fluidized powder 102 in a hopper 101 into the measuring tube 120 so as to accelerate the powder; detecting the pressure difference generated between the inlet and outlet of the measuring tube 120 by means of a pressure difference sensor 123 which constitutes a fixed small volume blind tube via a filter 122; obtaining an output signal by an amplified signal processor 124; detecting the powder flow rate by observing the indicated feed rate of the powder (g/min.) on a display device 125; and automatically controlling the driving gas 128, from an injector 129 comprising a throat 127 and a nozzle 126, by an automatic control means (not shown in the figure).
In the conventional art shown in FIG. 7, the sensing characteristics of the powder feeding rate is not affected by the variation of the fluidizing air flow rate, since the flow rate of the air accompanied by the fluidized powder is by 1/100 or less in comparison with the flow rate of the measuring gas 114. Accordingly, the characteristics do not show practical changes depending on the attached position of the hopper or the hopper itself. However, with respect to common powder materials, the working curve showing the relation between the feeding rate at a set value and the actually measured feeding rate varies in the range of about .+-.7.5% as shown in the working curves 130 and 131 in FIG. 8. This depends on the kind of the powder, or the variation of the mixing ratio of the fresh powder and recovered powder, as a result an on-site regulation is necessary. Incidentally, when the flow rate of the measuring gas 114 has to be reduced due to some circumstances at a working site, there is also a problem that the sensor output declines downwardly, and deviates from a linear relation, and then shows a non-linear characteristic in the region where the powder feeding rate is high as shown in the working curve 132.
In the above-described conventional art with reference to FIG. 7 and FIG. 8, the powder is merely introduced and accelerated by the measuring gas in the measuring tube. Therefore, the degree of powder dispersion differs depending on the kind of the powder, thereby causing a difference in acceleration, as a result different working curves are obtained depending on the kind and physical properties of the powder as shown by 130 and 131 in FIG. 8. The reason why a nonlinear working curve as shown by 132 in FIG. 8 will be explained as follows. A linear characteristic curve is obtained in the range where a powder flow rate is small because powder dispersion is sufficiently done. But when the powder flow rate is high, the powder is not dispersed so sufficiently that the acceleration efficiency decreases, thereby the working curve deviates from the linear relation and bends toward the x-axis with respect to the constant powder feeder which is controlled by an automatic control system in which the pressure difference generated by the powder flow is kept constant. Additionally, in the conventional art illustrated in FIG. 7 and FIG. 8, because of insufficient powder dispersion, the powder flow in the measuring tube is downwardly biased in the horizontally placed measuring tube by the influence of the gravity, thereby creating a large velocity difference between the powder flow and the measuring gas flow. In other words, it results in an insufficient acceleratinon of the powder flow. In this case, there is a problem that a working curve varies depending on physical properties of the powder or operational conditions.
The object of the present invention is to solve the problems in the conventional art summarized in FIGS. 4, 5, 6, 7 and 8.
Another object of the present invention is to develop an instrument for measuring the mass flow rate of powder which is capable of obtaining a constant linear relational output against the powder mass flowing per unit time (g/min.), that is, powder mass flow rate without the influence due to the kind and physical properties of the powder, thereby obtaining a sensor based feed-back system, or a constant powder feeder, for automatically controlling the output to coincide with a prescribed value.